How should RF transistors be biased?
RF transistors need DC bias delivered without disturbing the RF path. A bias tee (an RF choke or λ/4 line feeding DC, plus a DC-blocking capacitor in the RF path) is the basic structure. Active bias circuits hold quiescent current constant over temperature and device variation, and depletion-mode devices such as GaN HEMTs and many GaAs pHEMTs require a negative gate voltage applied before the drain supply.[1][2]
How is RF kept out of the supply?
- λ/4 high-impedance line with an RF short (capacitor or radial stub) at the far end, presenting an open at the device.
- Chip inductor with SRF above the operating frequency, plus a series resistor or ferrite for low-frequency damping.
- Multiple bypass capacitors covering different frequency ranges.
Why does the bias network affect linearity?
The impedance presented at the envelope (baseband) frequency to the drain or collector affects memory effects and IM asymmetry; a low impedance over the modulation bandwidth is desirable for wideband PAs. At the second harmonic, the bias feed is part of the harmonic termination.[2]
What is bias sequencing?
For depletion-mode devices, applying drain voltage with the gate at 0 V turns the device fully on and can destroy it. Sequence: apply negative gate bias, then drain voltage; reverse on shutdown. Many GaN PA modules include sequencing controllers.
Frequently asked questions
What is a bias tee?
A three-port network that combines DC and RF onto one port while isolating them on the others.
Why add resistance to a bias choke?
To damp low-frequency resonances and improve stability.
Do BJTs need temperature compensation?
Yes; VBE falls about 2 mV/°C, so fixed-voltage bias can lead to thermal runaway.
See also
References
Online sources accessed October 21, 2026; printed works cited by edition. See all sources.
- “Microwave Transistor Amplifiers: Analysis and Design, 2nd ed.”. G. Gonzalez, Prentice Hall. 1997.
- “RF Power Amplifiers for Wireless Communications, 2nd ed.”. S. C. Cripps, Artech House. 2006.